A number of processes are known for the preparation of 3,3',4,4'-biphenyltetracarboxylic acid salts (BPTA) which are important starting materials for the preparation of heat resistant polyimide resins. It is known that biphenyl compounds can be produced by reductive coupling of an aromatic halide. For example in U.S. Pat. No. 4,727,185, a method is disclosed in which a 4-halophthalic acid is condensed in the presence of a noble metal catalyst, a formic acid salt, and a small amount of alcohol. However relatively large amounts of expensive noble metal catalyst are required to obtain good yields and quality, as well as long reaction times. In U.S. Pat. No. 4,720,576 a process for preparing BPTA is described starting from 4-chlorophthalic acid using carbon monoxide in the presence of a supported palladium catalyst. The reaction requires pressures of 20-70 psig, large amounts of catalyst, and comparatively longer reaction times. Other references use reducing agents such as formaldehyde or methanol, but yields of final product are impractical for a commercial process.
It is known from U.S. Pat. Nos. 4,292,435 and 4,338,456 that biphenyl tetracarboxylic esters can be produced by oxidative coupling of orthophthalic esters by passing molecular oxygen through the molten compound in the presence of a noble metal catalyst. This process requires high temperatures (100.degree.-300.degree. C.) and can only be run to low conversion, necessitating the handling of a large recycle stream. It also requires the use of a diester as a starting material.